This invention relates in general to endoprosthetic devices and in particular to endoprosthetic wrist joints.
Wrist joints are commonly susceptible to fracture, rheumatic infection and metacarpal-radial arthritis, any one of which can severaly affect the ability of the wrist to perform its major functions of flexion, extension and rotation. What complicates the design of a suitable prosthetic replacement for an infected or damaged wrist portion is the complex osteoarthromuscular structure of the wrist and the heavy pressures to which a wrist is subjected during normal use.
A preferred endoprosthetic wrist joint is one which permits a full range of motion in every plane except rotational and includes a biomechanically (anatomical) accurate center of rotation. A full range of motion is one which is limited solely by the surrounding soft tissue of the anatomy rather than by the prosthesis structure. Anatomically centered means that the center of rotation is with respect to the anatomy of the wrist and not with respect to the stem axis of the prosthesis. Studies have shown that the anatomical center of rotation in the wrist is in the head of the capitate bone (ulnarward) and displaced slightly volarward. Providing a prosthesis with an anatomical center is important in that it permits restoration of an arthritic hand to its normal position. Patients with severe metacarpal-radial arthritis have hands which are pulled ulnarward and volarward. The reason for the arthritic deformity is that the deteriorated wrist has reduced the leverage of the dorsal and radial muscles in the hand. By using a prosthesis with an anatomical center, the dorsal and radial muscles regain their correct positioning and leverage and draw the hand back into a normal position.
A further consideration in the design of a suitable wrist prosthesis is the amount of motion in the rotational plane which is to be permitted, if any. Some wrist prostheses are arranged to prevent rotational movement. The calculated amount of rotation included in this design, however, does allow for small rotational displacements of the metacarpals with respect to the radius and this rotational displacement obviates stress being absorbed by one of the structural components of the prosthesis. This torque relief discourages cement loosening of the implanted component at the cement-to-bone interface. A still further design consideration is to allow the components of the prosthesis to dislocate from each other when loaded beyond capacity rather than break or loosen. Thus, such a prosthesis is able to be relocated by closed reduction which spares the patient the significant pain involved in reoperating to repair a broken or loosened prosthesis.
The following listed patents provide some indication of prosthetic wrist joints which have been conceived; however, none of these joints provide the various desirable features which have been mentioned above.
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 2,220,235 Langlais (French) 4/10/74 3,506,982 Steffee 4/21/70 3,837,008 Bahler et al. 9/24/74 3,909,853 Lennox 10/07/75 4,003,096 Frey 1/18/77 4,040,130 Laure 8/09/77 4,063,314 Loda 12/20/77 4,100,626 White 7/18/78 4,180,871 Hamas 1/01/80 ______________________________________
Langlais discloses a three-component prosthetic joint wherein motion in one plane is limited to the interface between the first and second components and motion in a different plane is limited to the interface between the second and third components. No rotational movement is permitted and the extent of movement in the first two planes is limited by the sides of the first and second components, respectively.
Steffee discloses an endoprosthetic joint wherein a modified ball and socket combination is employed. The socket opening is shaped so as to determine the limits of flexion, extension, hyperextension, adduction and abduction which the prosthesis shall be capable of approximating in usage. A key and keyway arrangement are provided to eliminate rotation about the longitudinal axis of the prosthesis.
Bahler et al. discloses an endoprosthetic wrist joint of a modified ball and socket combination. The generally spherical ball includes a radially protruding portion which is perpendicular to the longitudinal axis of the forearm and the generally spherical socket is provided with a slit-like guide to receive the protuding portion and prevent rotation about the longitudinal axis.
Lennox discloses an endoprosthetic wrist joint wherein the radial component includes a part-annular, spherically shaped socket and the carpal component includes a spherical ball with flatted sides to allow free entry into the radial socket. The carpal component further includes an elongated member which is slotted to receive a plurality of metacarpal components, each including a spherical ball slid into the slot.
Frey discloses a three-component endoprosthetic wrist joint which includes an intermediate component member which interfaces with the proximal component by a ball and socket arrangement and with the distal component by means of a V-shaped recess and corresponding protruding portion. Movement is only permitted in one plane, but at two different locations along the longitudinal axis of the joint, and the V-shaped recess can be asymmetrical of its center plane so that a greater degree of pivoting can occur in one direction rather than in the opposite direction.
Laure discloses a wrist joint prosthesis which permits vertical motion and sidewise motion but prevents twisting motion around the longitudinal axis of the forearm. The prosthesis includes a ball and socket arrangement wherein an intermediate member serves as the socket for the distal component and as the ball for the proximal component and each ball and socket combination includes a slot and an outwardly protruding portion to restrict movement in only one plane.
Loda discloses a wrist joint prosthesis formed by a ball and socket joint, from which stems are introduced into at least one of the metacarpal bones and the radial bone, opposite to each other. In one version, the distal stem portion also provides a secondary stem which is nailable into another of the metacarpal bones. The prosthesis does not prevent the free rotation of the distal stem portion.
White discloses an endoprosthetic wrist joint providing a universal pivot between the radial component and the metacarpal component. An object is to provide a joint of greater strength which is therefore not dislocatable under an overload stress.
Hamas discloses a endoprosthetic wrist joint with a single spherical pivot providing unlimited motion in all directions as well as rotation. Certain preferred offsets and ratios are also disclosed.
In view of those desirable features previously discussed and the absence of such features by the disclosures of these listed patents, it would be an improvement to the devices of such patents and to prosthetic wrist joints in general to provide a prosthetic wrist joint of a design which permits a full range of motion, has torque relief for a calculated amount of rotation, has a biomechanically accurate center of rotation and is able to dislocate when loaded beyond capacity rather than to break or loosen. The prosthetic wrist joint described herein provides each of these desirable features as well as other improvements as will be apparent.